[XFS] Fix dbflush panic in xfs_qm_sync.

The recent behaviour layer removal dropped the check for quotas that have
been requested at mount time but have subsequently been turned off. This
results in a panic when accessing m_quotainfo which has been freed.

This patch adds the check originally made by xfs_qm_syncall() to
xfs_qm_sync().

SGI-PV: 969769
SGI-Modid: xfs-linux-melb:xfs-kern:29908a

Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>